1. Field of the Invention
This application relates to the art of communication in sensor network constituted by nodes as communication terminals.
2. Description of the Related Art
“IEEE802.15.4” is one of the communication standards that has an upper layer of communication, which may be wireless, such as a MAC layer or a PHY layer. If a transmitting node transmits data or a command, the transmitting node transmits frames, such as data frames or command frames, to a receiving node in the order the request is received from the upper layer.
European Patent Application No. EP1657852 filed on Nov. 15, 2004, relating to CSL (Coordinated Sampled Listening) describes a system and method used for saving power during communication when a transmitting node transmits frames in synchronization with an intermittent reception timing of a receiving node. During the intermittent reception timing, the receiving node may receive frames. If the receiving node receives frames using the intermittent reception timing, the receiving node enters a startup state and then proceeds into a standby mode. If the receiving node will not receive frames, the receiving node enters a resting state to suppress an output of power from a battery. In this state, the receiving node cannot receive a frame, but power consumption is suppressed.
If the transmitting node transmits frames by the CSL method, there may be an appreciable amount of time lag between reception of transmission requests of an upper layer to the actual transmission of frames. If the transmitting node receives information of an established intermittent transmission timing of the receiving nodes, the transmitting node transmits the frames efficiently based on such timing. If however the transmitting node does not receive information of the established intermittent transmission timing, the transmitting node must search for such timing information so as to transmit the frames in accordance with the timing. The searching generally takes a lot of time compared to transmitting frames based on known intermittent transmission timing.
The CSL method also has a frame continuation transmitting function. When performing this function, the transmitting node may continuously transmit several frames to the receiving node. A frame includes a frame pending bit that indicates whether the frames are currently being transmitted on a continuous basis. If the frame pending bit corresponds to an “ON” state, the continuous transmission of the frames has started. If the frame pending bit corresponds to an “OFF” state, the continuous transmission of the frames has ended.
If frame transmission requests are continuously made for frames that will be sent to a single receiving node, each frame pending bit included in the respective requested frame will be in an “ON” state after frame transmission requests that have the same destination are made, and the frames are then transmitted. The frame transmission requests are prepared individually for each frame, so the frame transmission requests are a group of individually prepared frame transmission requests. The receiving node continues in a standby mode between transmissions of the frames so as to receive the frames successively without entering a resting state.
The frame pending bits of frames are turned into an “ON” state prior to transmission of the frames. The transmitting node transmits all frames that have the same destination, as a whole over a period of time based on the intermittent reception timing of the receiving node. Therefore, the time required to transmit the frames is shortened.
In the prior art, when the transmitting node is to transmit the frames, the order of transmission of the frames is first established. For example, a storage device included by the transmitting node may have a FIFO (First In First Out) buffer that stores the frame transmission requests temporarily. When operating with a FIFO buffer, the frames are transmitted in the same order as they are stored.
If the frames are transmitted by the above method, the following problems are caused. The frames that have identical destinations may need to be stored continuously in the FIFO buffer, so that the FIFO buffer can participate in the frame continuation transmitting function using the CSL method. However, the frame transmission requests are not always stored continuously in the FIFO buffer. Therefore, a case may occur where the continuous transmitting function cannot be properly used, and throughput decreases.
The following example illustrates when a failure to execute a continuous transmitting function is inefficient. A FIFO buffer may store a first frame transmission request that is to be transmitted to a receiving node A (this request is called A1), a second frame transmission request that is to be transmitted to a receiving node B (this request is called B1), and a third frame transmission request that is to be transmitted to the receiving node A (this request is called A2) in this order. Each of the receiving nodes A and B has an intermittent reception timing. In other words, nodes A and B each have repeating periods during which the node may receive a frame. For instance, node A may have a first receiving period and a second receiving period during which node A may receive frames. Similarly, node B may have a receiving period during which node B may receive a frame.
Both the first and second receiving periods of node A may occur before the receiving period for node B. Therefore, for maximum efficiency, the FIFO buffer may need to transmit frames corresponding to requests A1and A2 continuously to node A. However, the frame corresponding to the request B1 is stored in the FIFO buffer between the frames corresponding to the requests A1and A2 and therefore the FIFO buffer is not able to transmit the frames corresponding to the requests A1 and A2 continuously. Instead, the FIFO buffer must transmit the frames corresponding to the requests A1, B1 and then A2 in that order, which decreases efficiency since now the frame corresponding to request A2 is transmitted during a later receiving period than the second receiving period of node A. Thus, throughput decreases.
Node A has been described as receiving frames corresponding to requests A1 and A2 during different receiving periods. However, in some cases node A may receive frames corresponding to requests A1 and A2 during one receiving period. Thus, a receiving node may receive several frames during the same receiving period continuously.